1. Field of the Invention
Implementations of various technologies described herein generally relate to methods and systems for attenuating internal signal multiples in seismic data.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. Both the sources and receivers may be strategically repositioned to cover the survey area.
The seismic energy sources may propagate seismic waves into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer.
The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
The seismic waves that are detected by the receivers may include primaries and multiples. A primary may be a seismic wave that has reflected only once off an interface before being detected by the seismic receiver. A multiple, on the other hand, may be a seismic wave that has reflected off an interface more than once, i.e., multiple times. Multiples may include what are known as internal or interbed multiples. An internal multiple may be a seismic wave that has reflected back and forth between the top and bottom of a single formation, e.g., a rock layer, multiple times.
Multiples may represent noise in the recorded traces. As such, the analysis for determining the composition of the subterranean formations typically includes a process to attenuate the multiples. Internal multiple attenuation is a method of processing a recorded wavefield to remove internal multiples present within the recorded traces.
Various methods have been used for the removal of internal multiples from recorded traces. For example, internal multiples may be observed to occur at a particular time after a primary reflection where the time depends on subsurface layer thickness and trace offset. The offset may be the distance between the source and receiver. This fact has been used in various schemes to remove multiples.
Other methods involve complex ray tracing schemes which generate a synthetic multiple wave and subtract it from the actual wave to obtain a supposedly multiple free record. However, these methods are very awkward in that they require significant knowledge of the subterranean structure before the synthetic wave can be generated. Similar synthetic multiples can be generated using methods not directly involving ray tracing, e.g., field propagation techniques, but again these require detailed knowledge of the subsurface, and as such, are not as practical as would be desired.